There are many situations in which there is a need or desire to measure the rate of flow of a liquid or gas through a conduit. Because of the shape of the conduit, temperature differentials within the conduit and other factors, not all of the fluid may be flowing parallel to a centerline through the conduit. Rather the true flow direction at any specific location may be, and frequently is, at some angle relative to the centerline. In these non-axial flow environments it is possible to place a flow monitoring probe parallel to the centerline and obtain a flow rate. But depending upon the angle of the true flow direction relative to the centerline, the flow rate determined parallel to the centerline may be higher or lower than the actual flow rate. Thus, there is a need for a method which can accurately determine the true flow direction of a fluid flowing through a conduit.
The federal government of the United States has set limits as to the amount of pollutants that an electric utility or other business may emit into the air. Typically, these emissions are determined from measurements of the flow rate of the stack gasses through the stack and an analysis of the of the stack gasses to determine the levels of pollutants which are present. If one knows the flow rate and has another monitor which measures the concentration of pollutants in a selected volume of fluid one can calculate the quantity of pollutants emitted over any selected time period.
The United States has additional regulatory requirements which now require many electric utilities to continuously measure emissions of specified pollutants on a mass per unit time basis. Additionally, the continuous monitors must be periodically tested to assure that they are functioning properly. When such tests are done one must use a second "reference method" monitoring device. If the reference method monitor shows the continuous monitor to be reading low, the continuous monitor must be recalibrated. Adoption of these rules has put a new importance upon the errors which occur both in continuous monitoring and in the periodic reference method verification tests. Such errors can be very costly to both the supplier of the monitor and the utility. The supplier is affected because the reference method can erroneously indicate that the monitor is not meeting the performance guarantee. The utility is affected because it may have been reporting the emission of more pollutants than actually occurred. The new regulations establish monetary value in the form of trading credits to a measured ton of SO.sub.2 emissions. If the reference method is in error, that error will directly cause an enormous high or low use of the utility's SO.sub.2 allowance and SO.sub.2 trading credits. The value of such emissions is such that for large utilities as much as $1,000,000 per percent error in measured emissions may result.
In performing the required reference method verification tests the technician typically uses a type S (also called S-type) pitot tube made to specific dimensions. At each point the tester measures a differential pressure. The static pressure is taken at selected points. The differential pressure reading is then used to compute flow rate.
The test technician frequently assumes that the direction of flow rate of a fluid in a stack is along a centerline through the stack. Consequently, he orients the pitot tube parallel to the centerline to take his readings. However, in many, if not all, smokestacks the true direction of fluid flow usually is at some angle relative to the centerline. Further, this angle will in general be different at different measurement points. To obtain more accurate flow measurements the pitot tube should be oriented along the true flow direction.
Prior to the present method there has been no precise technique for finding true flow direction. Technicians who have attempted to find true flow direction have normally oriented the pitot tube in various positions, taken differential pressure readings and then used those readings to estimate the true flow direction.
Some technicians have attempted to determine the true flow direction by first finding a probe orientation where the differential pressure is zero. Such a direction is known as the null position. Once the null position is found the pitot tube is rotated 90.degree. from this position to the true flow direction. Attempts to find the null position have also been imprecise. Usually the null position is assumed to be half way between two consecutive positions which provide pressure readings of opposite sign.
After the technician has oriented the pitot tube at the measurement point he takes several readings at that point. Conventionally, the technician averages the pressure readings and then takes the square root of that average as the velocity or flow rate of the gas through the measurement point.
We have found that the techniques currently being used to position the pitot tube and the averaging technique used to compute flow rate are prone to error. Consequently, there is a need for a more reliable and more accurate method to measure true flow rate of a fluid through a conduit.